Known acoustic distance measuring apparatuses essentially comprise (see FIG. 1) an acoustic wave emitter E placed on the vessel N and emitting towards the bottom aiming for an acoustic responder S placed on the water bottom at said fixed point or near thereto. In response to the sound signals it receives, the responder S emits a pulse which is received by a group of hydrophones L1, L2 and L3 disposed underneath the vessel at corners of a polygon. By measuring the differences in time taken for the sound pulse to travel along the paths SL1, SL2 and SL3 between the bottom responder and the onboard hydrophones, and knowing at least one of the distances L1 L2, L2 L3, L3 L1 between the hydrophones or at least the depth H of the water, it is possible to determine the distance between a vertical line passing through the onboard emitter and a vertical line passing through the bottom responder. For more detail on the required calculations, reference can be made to the following publications:
System d'ancrage dynamique du PELICAN. Colloque GRETSI, Nice 1973, by Claude LEROY.
Localisation sous-marine precise dans les trois dimensions. Colloque GRETSI, Nice 1973 by Claude LEROY.
Acoustic measuring system and its performances. Preprints of the 1974 OTC Conference Vol 1 6-8 May 1974.
In other distance measuring apparatuses, an acoustic emitter/receiver E' is placed on the bottom of the vessel and it interrogates a plurality of responders A', B', C' disposed on the water bottom adjacent to the fixed point (see FIG. 2). By measuring the elapsed time differences over the paths E'A', E'B' and E'C' the same results can be obtained as above.
The above described apparatuses are unusable when the water is more than 3000 meters deep and the vessel itself is a considerable source of noise (e.g. drilling ship or platform).
The signal received by the, or each, onboard hydrophone is highly attenuated after a long path through the water, and is therefore capable of being drowned by the noisy environment such that unless suitable modifications are made to the apparatus, the signals become unusable.
A first proposed modification has been to increase the power of the, or each, responder. However, while a conventional type of responder operating at 25 kHz requires a power of 4 kw for operating in depths of 3000 meters, it will require a power of 2000 kw at 6000 meters, given that a sound signal is attenuated by 7 dB/km. Thus, relative to a source at 3000 meters, a source at 6000 meters provides a signal whose attenuation, expressed in dB, is 20 log (6000/3000) (for the divergence), increased by 7.times.3, giving a total of 27 dB. Providing the, or each, responder with a power of about 1000 kw is not practical, even in conditions where the installation is expected to operate for several months without interruption.
A second proposed modification is to use directional responders operating at a lower frequency (e.g. 10 to 12 kHz) for which the propagation attentuation is smaller, about 1.5 dB/km).
This solution is not desirable since it would require considerable development to get it operational, and requires a new type of responder to be manufactured, and in any case leads to a lower measurement accuracy.
Further, it would require existing hydrophones to be replaced which would make existing equipment obsolete.
Preferred embodiments of the present invention thus enable acoustic distance measurements to be made on noisy vessels and over great depths (more than 3000 meters and in particular up to 600 meters) without modifying existing equipment and without providing them with a large increase in power.